1. Field of the Invention
This invention relates to a dc power supply for supplying a dc power supply.
2. Description of the Prior Art
A dc power supply for supplying a dc regulated voltage from an input dc voltage with voltage control is known. FIG. 6 is a schematic circuit diagram of a prior art series regulator type of a dc power supply. In FIG. 6, a PNP type of transistor T1 is connected between an input terminal 2 and an output terminal 3 of the dc power supply 1, wherein an emitter is connected to the input terminal 1 and the collector is connected to the output terminal 3. Moreover, a smoothing capacitor C1 is connected between the output terminal 3 and a ground terminal 4. The transistor T1 is controlled by a power supply control IC 5. For example, in a dc power supply mounted on an automotive vehicle, an input voltage Vb is supplied through an ignition switch from a battery (these are not shown).
The power supply control IC 5 comprises a voltage divider 6, an operational amplifier 7, a driving circuit 8, a constant voltage supply circuit 9, and a constant current supply circuit 10. The voltage divider 6 detects the output voltage Vo through a voltage dividing structure. The detected output voltage Vd and a reference voltage Vr are supplied to the base of a transistor T2 and to the base of the transistor T3 forming a differential pair 11 in the operational amplifier 7, respectively.
The driving circuit 8 comprises transistors T4 and T5 connected to each other with a Darlington connection. Their collectors are connected to the base of the transistor T1. The transistors T4 and T5 operate in response to the error amplified voltage Ve from an output node n1 of the operational amplifier 7. Moreover, the capacitor C2 is provided for phase compensation connected between the output terminal 3 and the emitter of the transistor T5.
When the ignition switch is turned on and thus, the input voltage Vb (for example 14 V) rises stepwise, the constant voltage supply circuit 9 and the constant current supply circuit 10 immediately start their operations to output a predetermined constant voltage and a constant current. In response to this, the operational amplifier 7 and the drive circuit 8 starts their operations.
FIGS. 7A to 7C are graphical drawings showing output voltage variations on startup of the dc power supply according to the prior art, wherein the capacitor C1 is varied. FIG. 7A shows the output variation with the capacitor C1 of 6.8 xcexcF, FIG. 7B, 33 xcexcF, and FIG. 7C, 220 xcexcF. The dividing ratio of the voltage divider is 5:1, the reference voltage Vr is 1 V, the target voltage of the output voltage Vo is 5 V, and the output terminal 3 is connected to a resistive load of 12xcexa9.
At the startup of the power supply, that is, start of supplying the input voltage Vb, the output voltage Vo (the detection output voltage Vd) is 0 V. On the other hand, the reference voltage Vr is constant (1V). Then, the base voltage of the transistor T2 at the differential pair 11 of the operational amplifier 7 is lower than the base voltage of the transistor T3, so that the amplified error voltage Ve at the output node n1 increases. As a result, the base current to the transistor T1 flows through the transistor T5 of the driving circuit 8. This turns off the transistor T1, and thus, the output voltage Vo rapidly increases.
In this operation, if the output voltage Vo rapidly increases, the variation in the amplified error voltage Ve outputted from the operational amplifier 7 is delayed because the operational amplifier 7 cannot trace the variation of the output voltage Vo. As a result, the operational amplifier 7 cannot suppress the base current of the transistor T1 after the output voltage Vo reaches the target voltage, so that overshoot of the output voltage Vo occurs.
After this, the amplified error voltage Ve changes to descent from the ascent. In response to this change, the transistor T5 of the driving circuit 8 begins to decrease the base current of the transistor T1, so that the output voltage Vo begins to decrease. After the output voltage Vo decreases to the target of the output voltage Vo, the base current cannot be increased immediately due to the delay in the operational amplifier. As a result, under shoot is developed at the output voltage Vo.
Accordingly, the output voltage Vo converges to the target voltage with repetition of overshoot and under shoot at the startup of the power supply. In this operation, there is the tendency that the lower the capacity of the capacitor C1 the greater overshoot and the greater undershoot.
The aim of the present invention is to provide a superior dc power supply.
According to the present invention, a first aspect of the present invention provides a dc power supply comprising: a transistor for converting an input voltage into an output voltage on the basis of a driving condition; an error amplifier for generating an error signal on the basis of a difference voltage between a reference voltage and said output voltage; and a driving circuit for driving said power transistor on the basis of said error signal, directly detecting said output voltage, and controlling said driving condition to control a rise of said output voltage on at least a startup of said dc power supply.
According to the present invention, a second aspect of the present invention provides a dc power supply based on the first aspect, wherein said driving circuit suppresses a rate of said rise until said amplified error signal becomes in a steady condition after said startup.
According to the present invention, a third aspect of the present invention provides a dc power supply based on the first aspect, wherein said driving circuit suppresses a rate of said rise such that said amplified error signal can trace said output voltage.
According to the present invention, a fourth aspect of the present invention provides a dc power supply based on the first aspect, wherein said driving circuit includes: a voltage detection circuit for detecting a variation of said output voltage; and a driving control circuit for controlling said driving condition of said transistor on the basis of said variation detected by said voltage detection circuit.
According to the present invention, a fifth aspect of the present invention provides a dc power supply based on the fourth aspect, wherein said driving control circuit comprises a current mirror circuit for outputting a current determining said driving condition of said transistor in response to a current corresponding to said error signal, and said voltage detection circuit comprises a capacitor and a resistor connected in series, connected between said output voltage and a common control input of said current mirror circuit. In this case, the resistor in the voltage detection circuit can be omitted. That is, the voltage detection circuit may include only a capacitor.
According to the present invention, a sixth aspect of the present invention provides a dc power supply based on the fifth aspect, further comprising a delay circuit for keeping said transistor in an OFF condition for a predetermined delay interval after start of supplying said input voltage to said dc power supply.
According to the present invention, a seventh aspect of the present invention provides a dc power supply based on the sixth aspect, wherein said driving circuit further comprises a switch circuit for cutting off an output current of said current mirror circuit, and said delay circuit comprise an input voltage detection circuit for detecting start of supplying of said input voltage and a delay control circuit for controlling said switch circuit in said cutoff condition for said delay time from said start of supplying of said input voltage.
According to the present invention, an eighth aspect of the present invention provides a dc power supply based on the first aspect, wherein said error amplifier comprises a clamp circuit for accelerating a response of said error amplifier.
According to the present invention, a ninth aspect of the present invention provides a dc power supply based on the eighth aspect, wherein said error amplifier comprises a differential amplifier and said clamp circuit includes at least a diode connected to an output of said differential amplifier.
According to the present invention, a tenth aspect of the present invention provides a dc power supply comprising: a transistor for converting an input voltage into an output voltage on the basis of a driving signal; an error voltage detection circuit including an amplifier for generating an amplified error signal in accordance with a difference between a reference voltage and said output voltage with a predetermined delay to control said output voltage toward a target voltage; a driving circuit for generating said drive signal on the basis of said amplified error signal; and a surge control circuit for directly detecting said output voltage with substantially no delay and controlling said driving circuit to control rise of said output voltage on a startup of said dc power supply on he basis of said detected output voltage change.
According to the present invention, an eleventh aspect of the resent invention provides a dc power supply based on the tenth aspect, wherein said surge control circuit suppresses a rate of said rise until said output voltage reaches said target voltage on said startup.
According to the present invention, a twelfth aspect of the present invention provides a dc power supply based on the tenth aspect, wherein said amplifier amplifies said difference within a controllable range in a steady condition of said dc power supply, and said surge control circuit suppresses a rate of said rise such that said amplifier amplifies said difference within said controllable range on said startup.
According to the present invention, a thirteenth aspect of the present invention provides a dc power supply based on the tenth aspect, wherein said surge control circuit detects a variation of said output voltage.
According to the present invention, a fourteenth aspect of the present invention provides a dc power supply based on the tenth aspect, further comprises a delay circuit for keeping said transistor in an OFF condition for a predetermined delay interval after start of supplying said input voltage to said dc power supply.
According to the present invention, a fifteenth aspect of the present invention provides a dc power supply based on the tenth aspect, wherein said amplifier comprises a signal transistor for amplifying said error signal and a clamp circuit for clamping an output of said signal transistor to accelerate a response of said amplifier.
According to the present invention, a sixteenth aspect of the present invention provides a dc power supply based on the tenth aspect, wherein said detection circuit including resistive means and a capacitor, and said control circuit include a resistance to said capacitor as said resistive means.